It didn't run and was a bit beat up and abused, but looked great to me. The 1999 Husqvarna TE 410 was the bike that was new when I was first riding a 1991 Husqvarna WMX 610 back in the late 1990's, and I do remember seeing a new 1998 or 1999 model year Husqvarna four stroke at a dealership once. Back then I didn't know that it was the same bike as my 1991. People were saying that the later 1990's Husqvarnas were all new designs with zero parts interchangeability, and they looked very different at first glance. It turns out though that the differences are superficial. It's still mostly the same bike. The one I found for sale nearly 20 years later in 2017 looked fairly good with all of it's original plastic and the original seat cover still in serviceable condition. I was excited to see that the 1999 Husqvarna TE 410 had a Sachs shock on it, and the 45mm forks looked considerably different from the 45mm 1992 Showa forks also. After the bad experience I had with a 1992 Husqvarna a few years ago I was on the lookout for suspension differences.
Backfires but doesn't Run
Engine Rebuild
Slipping Magnets
Hard Starting
Improved but Unacceptable Suspension
Starting Tricks
Long Rod Mysteries
Pistons and Compression Ratios
With the 1994 and later centrifical de-compressor on the camshaft there doesn't appear to be any cranking compression when turning the engine over forwards. The centrifical de-compressor holds the exhaust valve open on the compression stroke at very low engine speeds so pushing down on the kick starter the engine feels like it has zero cranking compression. When I bought the 1999 Husqvarna TE 410 I wasn't worried that the cranking compression was non-existent, and I didn't bother to pull the ignition cover off to spin the engine over backwards to feel the cranking compression on the power stroke.
When I looked the bike over carefully after buyig it for $740 in cash I noticed some strange things about the 1999 Husqvarna and the 410 motor. The 1999 chassis looks very different with the bike fully assembled as the engine cradle is totally different than the 1986 through 1998 Husqvarna four stroke frames. The 1999 model has just two frame tubes running under the engine where the 1986 through 1998 frames have three frame tubes running under the engine. The 1999 frame is also painted dark gray, where the 1998 and earlier frames are painted white. The rest of the 1999 frame is very similar to the 1998 and earlier frames, but there are a few minor differences in joinery. It is still the same big 57mm main tube with the same shock tower mount and the same engine mounting points. The 1999 frame is just welded together a bit differently in some places. Just about everything is still interchangeable with the older Husqvarna four strokes. The 1999 sub frame, seat, air box and rear fender are interchangeable with the 1992 through 1998 Husqvarna four strokes and nearly identical other than color changes. The 1999 seat is however a lot taller and softer than the 1992 seat, a significant difference. The 1999 exhaust system is nearly identical to the 1992 exhaust system, but the mufflers are Husqvarna brand where the 1992 mufflers were J&R Answer brand. They are the same size though with the same mounting points and are still prominently stamped with "USFS Approved Spark Arrestor".
The 1999 Husqvarna TE 410 motor looks exactly like the 1997 Husqvarna TE 610 motor I have. The only difference that can be seen from the outside is that the cylinder is shorter. It is exactly the same cylinder height as on the 1991 and 1992 Husqvarna 350 motors. That caught my attention right away because the 410 has a shorter stroke length than the 350 by a rather significant 2.2mm. That means that the crank pin is 1.1mm lower at top dead center on the 410 motor than on the 350 motor.
This 1999 Husqvarna TE 410 had obviously not been ridden in quite some time. The chain and stock size 13/48 sprockets were apparently in perfect condition, but the chain was rusty and there were prominent rust marks on the aluminum sprocket from sitting in contact with the rusty chain out in the elements. The frame was a bit rusty in places where the original paint had been abraded off. Not just old worn in rust, but fresh fluffy rust like only develops when something sits stationary for long periods of time. I was impressed with the overall durability of the slippery Teflon based coating on the 1999 frame. It seemed very durable, but this bike had been ridden extensively enough that there were places where the coating had come off.
The first problem with the 1999 Husqvarna TE 410 was that the 36mm DellOrto carburetor was in sad shape. The gasket on the bowel was torn and gasoline just poured out. When I pulled the main jet out I found an even worse problem, it had been drilled out to the 180 size. The 180 size is the stock main jet size for the 40mm DellOrto on the early 1990's Husqvarna 610 motors, way too big for the smaller 36mm DellOrto carburetor. It was a 170 jet that had been drilled out to the 180 size. When I looked up the stock main jet size for the 36mm DellOrto on the 410 motor it was listed as a 170. That also is way too big for the 36mm DellOrto. Something strange here for sure.
The first thing I had to do was guess at the correct main jet size for a 36mm DellOrto carburetor. The 34mm DellOrto on the 1991 Husqvarna 350 comes stock with a 145 size main jet, and the 36mm DellOrto is much closer in size to the 34mm DellOrto than to the 40mm DellOrto. Obviously the 36mm DellOrto would take a main jet closer to 145 than to 180. My guess was about a 155 size for the 36mm DellOrto. Since I didn't have a 155 size main jet, or any smaller jets to drill out, I had to solder up the stock 170 main jet and drill it out smaller. With the drills that I had what I got was more like a 157 size main jet, which seemed just fine. If the stock main jet size for the 36mm DellOrto was listed at 170 then it would be hard to imagine a 157 main jet as being too big.
The 36mm DellOrto on the 1999 TE 410 had the stock listed 45 starting jet but the pilot jet was one size smaller than the stock 60. It had the 58 size pilot jet listed for the 34mm DellOrto. Since I have been running a 60 size pilot jet in the 40mm DellOrto on my hot rod 610 motor it seemed perfectly reasonable to go down to the 58 size pilot jet on the 36mm DellOrto. Obviously the pilot jet circuits are not proportional in size to the size of the carburetor. The 34mm DellOrto, the 36mm DellOrto and the 40mm DellOrto all have rather similar size pilot circuits and take rather similar size pilot jets.
I wasn't sure where to set the needle clip position on the 36mm DellOrto. On the 34mm DellOrto the first needle clip position is still a bit on the rich side, where on the 40mm DellOrto the 1st needle clip position is rather lean. On the 40mm DellOrto I can never decide whether to run the needle clip in the 1st or second position. The 1st position works great, but on unusually low energy density gasoline it can easily seem too lean. The 2nd needle clip position on the other hand is just always too rich for any type of gasoline. On the 34mm DellOrto though the 1st needle clip position is always best. I wasn't sure how the 36mm DellOrto was setup, so it was just a guess as to where to set the needle clip position. I decided to start with the leanest position since that is where I have been running the clip in all of my other DellOrto carburetors. It is the same K32 needle that the 34mm DellOrto and the 40mm DellOrto have in them.
The carburetor bowel gasket I just glued on with RTV silicone as I didn't have a replacement handy. By letting the silicone fully set up for several hours before adding gasoline the glue held.
With the carburetor back together I tried to start the motor. Nothing. I pulled the spark plug out and watched for spark. The ignition system was firing. Even when I pushed the kick start lever down with my hand I very easily got a spark. The engine wouldn't start though. After kicking about 10 times I pulled the spark plug out again, and it was wet with gasoline.
I tried rolling the bike down a little hill, but it still wouldn't fire up. What it did do when rolled though was backfire very loud out the exhaust.
I pulled the ignition cover off to feel the cranking compression, and there wasn't any. Turning the engine over backwards on the power stroke was the same as turning the engine forward on the compression stroke. No cranking compression at all, so I tore the motor apart.
When I got the cylinder off I found out right away what the problem was. The compression ring fell out of the piston in about two dozen little pieces. Wow, that's a broken ring and then some! What was really surprising though was that the cylinder was not badly damaged. The piston was in reasonably good condition also.
The cylinder actually looked pretty bad, but the damage was not deep. There were patches of discoloration from corrosion around the cylinder about 3/4 of the way down where the piston had been sitting when water got into the cylinder. These discolored patches looked bad, but I could hardly feel them with my fingernail. When I measured the depth of the damage with an inside micrometer I found that it was less than 0.0002" deep. The taper and out of round were also very small. Essentially a new engine, just with a severely broken compression ring.
How had the compression ring gotten broken? Obviously it had to do with the slight corrosion damage. The ring had stuck to the cylinder wall and then broken when the engine was later turned over. But how had the ring broken so easily when the damage to the cylinder walls was so slight? I held a larger two inch long piece of the broken compression ring between thumb and forefinger and twisted. It broke unexpectedly easily. A very weak cast iron ring. It had worked fine, but when it got a bit stuck to the rusty cylinder wall it had just shattered into dozens of little pieces.
When I looked at the cylinder head I was surprised and very disappointed to find that it had the same small valves as the 1991 through 1993 Husqvarna 350 motor. The 410 has the 91.5mm bore diameter of the old 510 motors, and I had been expecting the same 35mm intake valves and 30mm exhaust valves as the 510 motors. Nope, they used the small 32.5mm intake valves and 27mm exhaust valves from the 350 motor. Why such small valves? That's ridiculous. To cram the same cylinder head onto the small 84mm bore of the 350 motor smaller valves were required, but the 91.5mm bore of the 410 and 510 motors have room for considerably larger valves.
The compression height of the OEM flattop piston in the '99 410 was 24.5mm, which seemed very short. When I stuck the piston and cylinder back on the engine with the stock 0.050" thick base gasket the top of the piston was up within 0.012" of the top of the cylinder. Something strange here for sure.
The 1999 Husqvarna TE 410 head and base gaskets were the same 0.055" and 0.050" thickness as all of the Husqvarna 610 head and base gaskets. I even checked the part numbers in a 1999 Husqvarna parts catalog, and the 410 and 610 motor take the exact same base gasket. Next I turned to the bottom end.
With the oil washed out of the rod bearing I could feel a slight bit of radial play, probably less than 0.0015". Essentially perfect. The connecting rod looked pretty much just like the 127mm rods in the 350 and 610 motors. When I looked at the big end shell though there was a difference. The 1999 big end shell was a stepped design, with a 3/8" wide ridge sticking up in the middle. The outside diameter of the central ridge is rather large at about 2.24", which is about 0.18" bigger than the rod bearing shell on the 1990 through 1997 Husqvarna 350 and 610 rods. That is only that 3/8" wide central ridge though. On the outside parts the 1999 connecting rod big end shell is actually about 0.15" smaller in diameter than the 1990 through 1997 Husqvarna 350 and 610 connecting rods. Very strange. The strangest thing though was that the 1999 connecting rod appeared to be longer than the 1990 through 1997 connecting rods. Longer by about 4mm, although it was hard to get an exact measurement with the crankshaft installed in the cases. The loner rod explains the short compression height of the piston.
The compression ratios of the 1994 through 2001 Husqvarna 410 motors are variously listed at 11.1:1 or 11.4:1. When I roughly measured the volume of the combustion chamber on the 1999 Husqvarna TE 410 I was getting a compression ratio of just 10.3:1, much lower than the advertised compression ratio. Lots of strange things about this 410 motor.
To add to the confusion about the 410 motor there were three different Woessner pistons listed for different years of the 410 motor. The 91.5mm Woessner 8518DA piston for the 1999 through 2001 Husqvarna 410 motors was listed as having a 25mm compression height, rather close to the 24.5mm compression height of the OEM piston that came out of the '99 410 motor. Woessner also listed a 91.5mm 8515DA piston with a 27.8mm compression height for 1993 through 1995 Husqvarna 410 motors and a 91.5mm 8514DA piston with a 29.5mm compression height for 1996 through 1998 Husqvarna 410 motors. Those are much taller compression height pistons that simply would not fit in the 1999 Husqvarna 410 motor without drastic modifications. Even the 27.8mm compression height piston would interfere with the cylinder head by a whopping 0.050". Obviously these taller compression height pistons go with the shorter 127mm center to center rod length of the 1990 through 1998 350/410/610 connecting rods. That means there is a high compression piston and a low compression piston for the early Husqvarna 410 motors, but for the 1999 Husqvarna 410 motor there is only one piston available.
All that my 1999 Husqvarna TE 410 motor really needed was a new ring set, but new rings weren't available. The OEM rings are obviously thicker than the Woessner rings, so there isn't any interchangeability. I actually did find an OEM ring set for the 1999 Husqvarna TE 410 listed for sale, but the price was $260 just for the rings. A Woessner piston kit was only $170 delivered, so buying just the rings didn't make any sense at all.
The Woessner pistons often are available in two very small oversizes, and the 8518DA was no exception. The DA ending indicated the standard 91.44mm diameter to deliver a 0.06mm(0.0024") skirt clearance in the stock 91.50mm bore. There was also an 8518DB piston listed at a 91.45mm diameter and an 8518DC piston listed at a 91.56mm diameter. My cylinder was very close to the stock 91.50mm diameter, so the smallest piston was the correct one.
I ordered the Woessner 8518DA piston, and when it arrived it was very close to the same 24.5mm compression height of the OEM piston and it did measure 91.44mm to the outside of the soft black coating on the skirts. The Woessner piston was however lighter, which I was very pleased to find.
The OEM 1999 Husqvarna 410 piston is actually a pretty nice piston itself and weighs only 274g. The Woessner 8518DA piston was considerably lighter at 265g, and the Woessner pin was also a nice tapered design that saved 12g over the irrationally heavy 79g OEM pin. The stock Woessner 8518DA piston kit saved 22g in weight, a nice upgrade.
With that huge heavy connecting rod it seemed like a good idea to get the piston down as light as possible, so I decided to cut some off of the new Woessner piston. The 91.5mm Woessner piston is exactly like the 98mm Woessner pistons, just smaller. The shapes are all the same, so the strategy for cutting it down was very similar to what I did on the 98mm pistons before. With little difficulty I had it down to 243g, an additional 22g weight savings.
I also cut some material off of the small end shell on the connecting rod. The 1999 small end shell was rather large, and with the small 20mm piston pin there was obviously a lot of extra material. It was about 10g that I got off the small end shell to bring the weight of the longer rod down very close to the 313g weight of the 1990 through 1998 Husqvarna rods. It is a hugely oversized rod for the 2.39" stroke length 410 motor, so I could have cut even more off if I had the rod out of the engine. With the rod installed in the engine though I was happy just to get the small end shell down a bit lighter.
I poured some gasoline down the intake and exhaust ports to check for valve seat leaks, but it looked pretty good. A very small amount of the gasoline did come through when I let the cylinder head sit for 10 minutes, but the leaking looked even all around and about the same on all four valves. It was just enough gasoline coming through to wet the area within about 1/16" from the seat. No sign of any trouble, so I didn't bother to pull the valves out of the head.
Instead of honing the cylinder I just cleaned it up a bit with 600 grid silicon carbide paper. I smoothed in the entire cylinder, but particularly around the area of the corrosion damage I worked it in substantially to take any sharp edges off. The 600 grit paper takes very little material off, but it was enough to get the cylinder looking mostly uniform.
I reused the old OEM gaskets, which looked just like the 1990 through 1998 gaskets. With a light coat of RTV silicone on all surfaces the engine went together well.
When I was taking the engine apart I had roughly checked the cam timing, and it looked like the camshaft was advanced by about 4 degrees of crankshaft rotation. The 1999 TE 40 camshaft looked a lot like the 1994 and 1997 Husqvarna TE 610 camshafts, but the '99 TE 410 camshaft was actually smaller. The duration was about 8 degrees of crankshaft rotation shorter. Since the '99 TE 410 camshaft was smaller it seemed like it would be better installed less advanced. I noticed that there was some substantial wiggle in the mounting of the camshaft sprocket, so on installation I held the sprocket so that the cam timing would come out less advanced while I torqued the bolts down. This did seem to make a significant difference, and the cam timing was looking more like 1.5 degrees advanced. Much more appropriate for the small camshaft.
With the 410 motor back together the cranking compression was all the way back up. Turning the engine over backwards on the power stroke the cranking compression was perfect right from the start. The engine still wouldn't fire up though. Just nothing. I was still getting spark, and the spark plug was still coming out wet with gasoline, but it wouldn't start.
When I rolled it down a little hill I got more backfiring than I had before the rebuild. After a while I was getting backfiring sometimes when I kicked the engine also. Very strange. The spark timing appeared to be dramatically incorrect.
The ignition system on the 1999 Husqvarna TE 410 is the same Ducati Energia CDI system that came on the 1997 Husqvarna TE 610 motor. I hadn't been able to find a flywheel puller back in 2015, but this 1999 Husqvarna TE 410 had come with a box of spare parts that included a flywheel puller.
The flywheel nut looked a bit rusted on. It hadn't been apart in a while. The flywheel did come off without much difficulty though. There was a key in the key slot, so the flywheel hadn't been spinning on the crankshaft. The stator was set about in the middle of the adjustment range, so the spark timing should have been at least sort of close to correct.
There was some mud and dust on the stator and in the flywheel, so I cleaned it all up and put it back together. The bike still wouldn't fire up. It was exactly the same, lots of loud backfiring but it wouldn't start. It seemed like the spark timing was dramatically far off, but it took me a while to figure out why.
I hooked the stroboscopic timing light up to the engine and rolled it down the hill trying to point the light at my timing marks. That didn't seem to work, I couldn't see anything.
Then I found that I could spin the engine over with a 1/2" drill motor with the spark plug removed. This did the trick, and I was able to watch the spark timing with the timing light.
What I found was very strange. The spark timing was different each time I spun the engine up. The flywheel is bolted down solid, but the spark timing is jumping all over the place. The clue I got about what was going on was that the spark timing was coming back to the same values. Sometimes it was at 120 degrees BTDC, sometimes at 90 degrees ATDC, sometimes at BDC. It was jumping all over the place, but in 30 degree increments.
Finally after some considerable head scratching I realized that the magnets were spinning inside the flywheel. The magnets were somewhat loose when I pulled on them. So that was the original problem that caused the bike to get stored outside all those years ago. First the ignition system failed, and then the bike got kicked outside where a bit of water got in the cylinder and damaged the engine.
I put the flywheel off of the 1997 TE 610 motor onto the 1999 TE 410 motor and it fired right up on about the third kick with the choke on.
The 410 motor ran well, but it wouldn't restart. I shut it off before it was even fully warmed up after just a very short one mile ride at small throttle openings, and it wouldn't fire up with the kick starter. Just nothing. With or without the chokeit wouldn't kick start at all. When I rolled though it fired right up. I rode around a bit more and then changed the oil, and again it wouldn't start with the kick starter.
The 410 motor seemed to be running quite well with nice easy to modulate instant torque over a wide range of engine speeds without any surging or whining. Torque seemed pretty good, a lot more than just 4% more than the 386 stroker motor. The 410 motor also revved out willingly, and the power seemed to build nicely. It wouldn't restart though.
I tried a half dozen times to restart it with the kick starter, and it just wouldn't fire at all. I took the spark plug out and cleaned it with a propane torch, and then the motor fired right up rather easily with the kick starter. After another short little one mile ride it again wouldn't kick start. The suspension was not good either, and that was very disappointing. Bad suspension is even worse than hard starting.
I tried gapping the spark plug down to 0.011", but that didn't help. It still wouldn't kick start. Cleaning the spark plug with the propane torch did however get it to kick start each time I tried this. That's better than having to replace the spark plug each time to get it started, but not much better.
The 410 motor ran very consistently and was perfectly stable at elevated low idle speeds, but then when the low idle dropped down it got very unstable. The low idle speed would drop down all of a sudden, and it dropped down quite a lot. At the lower low idle speed there was some cutting out and stumbling, and it would often stall if it stayed down there low for more than a few seconds. Turning the idle stop in to raise the low idle didn't make much difference. It just prevented the low idle from dropping down as quickly. If left idling it would still eventually drop down to the lower low idle speed and stumble and stall. If I turned the idle stop in far enough that the 410 motor didn't drop down at all then it seemed to idle up very high.
When I put a timing light on the Ducati Energia CDI ignition system I found that it has three spark timing values. The cranking and low idle spark timing value, the running spark timing value and an in-between high idling spark timing value. The cranking and low idling spark timing value was at about 11 degrees BTDC, and the running spark timing value looked like it was at about 23 degrees BTDC. The 11 degree BTDC low idle spark timing value was only down rather low, bellow about 1,600RPM or so. I didn't have a tach on the engine, but it seemed way down very low. The in-between spark timing value was about 20 degrees BTDC, and that only occurred over a rather narrow range of engine speeds up to perhaps 3,000RPM or so. Then it was just the one fixed 23 degree BTDC running spark timing value all the way out to high engine speeds.
It was easy to read the 11 degree BTDC low idle spark timing, as that was rock solid and not fluctuating. My Actron L100 timing light was however acting up a bit at the running spark timing value. It was obviously just one fixed spark timing value at all engine speeds above about 3,000RPM, but the light was jumping around a bit as the Actron L100 sometimes does. It was jumping up to 26 degrees BTDC, but it was also down at 23 degrees BTDC a lot of the time. I am fairly certain that the actual spark timing is down at 23 degrees BTDC.
When I put the same timing light on the stock 10.2:1 Czech Republic CDI equipped 610 motor it reads 26 degrees BTDC, but jumps up to 29 degrees BTDC sometimes before stabilizing at 26 degrees BTDC again. In any case the 10.2:1 Czech Republic CDI equipped 610 motor is running about three degrees more spark advance than the 1999 Husqvarna TE 410 motor. And this is what would be expected since the 610 motor has a larger bore and perhaps a slightly lower compression ratio also.
The Ducati Energia CDI ignition system on the 1999 Husqvarna TE 410 was seeming to deliver dramatically lower spark energy at the low idle spark timing value. Even once fully warmed up the engine would get very unstable as soon as the engine speed dropped down there to where the spark timing was at 11 degrees BTDC. Actually it seemed pretty much the same hot or cold. Whenever the low idle speed dropped down there to the 11 degree BTDC engine speeds the engine got very weak sounding and would usually stall in short order.
Up above low idle though the ignition system seemed to work fine, and performance was fairly good. I never noticed any undue harshness at any engine speed, and torque was seeming fairly good at all throttle openings over a rather wide range of engine speeds.
That was the first day. The next morning the gasoline was very different. The 410 motor still wouldn't kick start, but it didn't run as strong either. On the weaker gasoline the 410 was requiring much larger throttle openings to make torque, and it seemed unwilling to rev out. The power delivery was also worse, with some little hints of surging developing around 6,000RPM or so. The 410 motor was still rather crisp down low around 3,000 to 5,000RPM, but to get going and make midrange torque it was taking a big twist of the throttle and then the torque didn't build like it had before. Very disappointing.
For several days I rode the 1999 Husqvarna TE 410 around on short little test rides. Each morning the gasoline was different, sometimes dramatically low energy density to where there was actually cutting out and stumbling at around 4,000RPM when the engine was cold and torque was very low with some surging once fully warmed up. Other days the gasoline was more normal, but always dramatically weaker than the first day when the 410 motor had actually seemed to run well.
When I swapped the same gasoline back and forth between the 1999 Husqvarna TE 410 and the 9.7:1 386 stroker motor the bigger bore 410 ran more crisply than the 9.7:1 386 stroker motor did with a 21 degree BTDC static timing setting. On the same gasoline the 410 motor was rather crisp even at small throttle openings, where the 9.7:1 386 stroker motor was needing a sizeable twist of the throttle to get going. The 386 stroker motor was able to rev out and make power without cutting out, but it does have that three degrees of crankshaft wiggle advance that comes on above about 7,000RPM.
This was obviously slightly higher pressure gasoline than I had been getting in recent months. For quite a few months the 9.7:1 386 stroker motor has been running extremely crisply, overly crisp really, with the same 21 degree BTDC static timing setting. On the gasoline out of the 410 the 9.7:1 386 stroker motor was needing to be fully warmed up on some big pulls before it was seeming crisp enough. It was running and making power, but it was more reluctant to get going.
The 410 motor is running a bit more spark advance bellow 7,000RPM, but the 91.5mm bore is also substantially larger than the 85mm bore on the 386 stroker motor. The 410 motor certainly does have a higher compression ratio than the 386 stroker motor, but it doesn't seem dramatically higher. The stock compression ratio of the 1999 Husqvarna 410 motor probably is closer to 10.3:1 than the advertised 11.4:1 compression ratio.
When swapping the gasoline back and forth between the 410 motor and the 386 stroker motor there were some pretty big difference in power delivery. One was a near total lack of surging in the 2.39" stroke length 410 motor. On the same gasoline the 2.68" stroke length 386 stroker motor was surging quite severely everywhere from 5,000 to about 6,500RPM. It still made some torque and was able to rev out , but there was just a substantial amount of surging. The same weak gasoline in the 410 motor caused only the barest hint of surging starting to develop around 6,000RPM.
The other difference was of course the amount of torque. The 410 motor makes a lot more torque on the weak gasoline. That first day on normal gasoline I was quite impressed with the torque that the 410 motor was able to make over a wide range of engine speeds. Then even on the dramatically weaker gasoline there was a lot more torque from the 410 motor than from the 386 stroker motor. On weak gasoline the 11% shorter stroke length of the 410 motor is a huge advantage, both in quality of power delivery and in quantity of torque. The 410 motor is only 3.6% more displacement, but the increase in torque was much more substantial. Interestingly the 386 stroker motor also seemed to pull hard over only a rather narrow range of engine speeds, where on the same gasoline the 410 motor felt quite a bit broader.
Somewhat unexpectedly I was finding that I actually had to rev the 386 stroker motor up quite aggressively to get substantial power, where the 410 had a more immediate power delivery that got the bike moving along with less effort.
On the first few little test rides I actually thought the 1999 Husqvarna TE 410 suspension might be fairly good, at least it was obviously a lot better than the 1992 Husqvarna Showa suspension. I was optimistic about the low speed compression adjuster on the Sachs shock, and it did actually seem to work. Turning the low speed compression clicker all the way out resulted in a floppy and very busy rear end that bottomed easily. Going in on the low speed compression clicker firmed things up dramatically, and the rear end was able to stay up over rolling bumps and was also able to handle landings.
As it turned out though that low speed compression clicker was all that worked on the 1999 Husqvarna TE 410 suspension. And even at that there was a problem. With all of the clickers all the way out the rear end of the '99 TE 410 got quite plush and comfortable, but without low speed compression damping it was extremely loose feeling and bottomed out when there didn't even seem to be bumps in the trail. Turning the low speed compression clicker in just a few clicks firmed up the rear end very substantially with fairly good hold up on rolling bumps and on jump landings, but it also got harsher. The problem is that turning the low speed compression clicker in made the rear end harsher than on the 1999 Husqvarna WMX 610 suspension that has even better hold up over rolling bumps and on jump landings. So the low speed compression clicker does work on the 1999 Sachs shock, but the excess harshness won't go away. Backing out all the way on the high speed compression clicker on the shock did help a bit, but only when the low speed compression clicker was also all the way out. Even with the high speed compression clicker all the way out going in on the low speed compression clicker to get some hold up seemed to dial in an unexpectedly large amount of harshness. Both ends were harsh feeling, although the front was much worse than the back. The 1999 Husqvarna Sachs shock seems very close to functional, there is just that little bit of undue harshness.
The more I rode the 1999 Husqvarna TE 410 the worse the suspension seemed. The first thing I tried with the forks was to back out on the compression clickers to try to reduce the extreme harshness over sharp bumps at medium to higher speeds. That didn't work. Even with the compression clickers all the way out the front end still got very harsh over sharp bumps at somewhat elevated speeds. At low speeds the forks were fairly plush and comfortable, and seemed to soak up small bumps well over a range of speeds. When the pace picked up a bit though the front got pretty harsh over certain types of bumps. It was the first impact with sharp bumps that felt so harsh, as the suspension compressed further over larger bumps it worked better. This harshness was not however only at the top of the stroke. Under heavy braking with the forks compressed they were even worse, just extremely harsh feeling when hitting sharp bumps.
The harshness was what I was most annoyed by. I didn't want a bike that was harsher and less comfortable to ride. There are however other severe problems with the 1999 Husqvarna TE 410 suspension. It is much faster than the 1992 Husqvarna Showa suspension, but it is still slower than the 1991 Husqvarna WMX 610 suspension by quite a lot. The 1999 TE 410 suspension feels bouncy, and the rebound clickers don't do much of anything.
I tried going all the way out on the rebound clicker on the Sachs shock, and not much changed. The rear end still felt like it had sufficient rebound damping, it was just a bit harsh and bouncy. The front end is much worse. When I went all the way in on the rebound damper the front end did stay lower, but it still felt bouncy. Particularly landing from jumps the front end had a very insecure feeling, like the front tire was trying to squirt out to the side. Backing all the way out on the rebound damping on the forks resulted in only a very slight change. With the rebound clickers all the way out the front end came up noticeably faster, but it still didn't work. It still felt bouncy.
The compression clickers on the forks did even less. With the compression clickers all the way out the harshness dropped off only very slightly. With the compression clickers all the way in the forks didn't compress as far on small landings, but the front end still felt bouncy.
Basically what it comes down to is that the only clicker that actually does much of anything on the 1999 Husqvarna TE 410 is the low speed compression damping on the Sachs shock. The other clickers don't do much, and the slight change that they do make seems worse in both directions. No matter what I did with the clickers the 1999 Husqvarna TE 410 suspension just felt worse.
Perhaps the worst functional problem was the front end instability that the 1999 forks were causing when landing from even small jumps. As the forks were compressing on a landing something seemed to be wrong, and the front tire felt like it wanted to squirt out to the side. The 1999 Husqvarna TE 410 sits with quite a lot of front end bike sag, about 1-3/4". That's way more front end bike sag than the 1-1/4 to 1-3/8" that I run on the 1991 Husqvarnas. The 1999 TE 410 sits with lots of front end bike sag, but when riding the fork springs feel rather stiff as the forks compress. Part of this front tire squirting out to the side problem might be insufficient pre-load on the fork springs. Or there might be something wrong with the springs themselves. I think that most of the unstable feel with the front tire wanting to squirt out to the side is due to incorrect damping. The springs might contribute to this problem, and the bald front tire certainly contributes to this problem, but most of what I am feeling is due to inferior damping.
The 1999 Husqvarna TE 410 suspension is faster than the 1992 Husqvarna Showa suspension, and it isn't quite as harsh either. The 1999 Husqvarna TE 410 suspension is an improvement over the 1992 Husqvarna Showa suspension, but not enough. It still doesn't work. I can go somewhat faster on the 1999 Husqvarna TE 410 than the 1992 Showa Husqvarna, but it is uncomfortable and unpleasant to ride. The 1999 Husqvarna TE 410 suspension is still a lot slower than the 1991 Husqvarna WMX 610 suspension while also being a lot harsher and more uncomfortable than the 1991 Husqvarna WMX 610 suspension. I would chose the 1999 Husqvarna TE 410 suspension over the 1992 Husqvarna Showa suspension, but that isn't saying much. It still stinks.
At least the 1992 Husqvarna Showa suspension does one thing well, it stays low over jumps on a motocross track. That's not much, but it's something and it makes the 1992 Husqvarna Showa suspension competitive on certain types of well groomed tracks where bump absorption is of secondary importance. The 1999 Husqvarna TE 410 suspension does many things better than the 1992 Husqvarna Showa suspension, but it doesn't seem to do anything all that well. Compared to the 1992 Husqvarna Showa suspension the 1999 TE 410 suspension has a lot more impact absorption capability and also doesn't pack up over successive bumps. Those are big improvements, but not enough. Compared to the 1991 Husqvarna WMX 610 White Power suspension the 1999 Husqvarna TE 410 is very bouncy and uncomfortable and won't go as fast.
Riding the 1999 Husqvarna TE 410 and my 1991 Husqvarna WMX 386 on the same day on the same trails with the same gasoline was both a good motor comparison and a good suspension comparison. When flogged hard through the surging and up into the power the 386 stroker motor was putting down enough power to get the bike going fast enough for a good comparison. No matter how fast I went the 1991 WMX 610 White Power suspension felt ready for more, where the 1999 Husqvarna TE 410 suspension felt bouncy and overpowered even when I kept the throttle nearly fully closed and just cruised along. There were two really big differences between the 1991 WMX 610 White Power suspension and the 1999 TE 410 suspension: Comfort and stability. Over any type or size bumps at any speeds above about 10mph the 1991 WMX 610 suspension on the 1991 Husqvarna WMX 386 delivered a dramatically smoother ride than the 1999 TE 410 suspension. The harshness on the 1999 TE 410 not only made for an uncomfortable ride, but it also hurt stability. Both going uphill and downhill the 1991 WMX 386 felt extremely stable over all types of bumps, where the 1999 TE 410 bounced around felt difficult to manage. Perhaps the most dramatic stability difference between the 1991 WMX 610 White Power suspension and the 1999 TE 410 suspension was in landing from jumps. The 1991 WMX 386 just touched down smoothly with no bouncing up and a very planted feel. The 1999 TE 410 on the other hand felt out of control landing from jumps, and any time the bike got a bit sideways I had to struggle to keep from crashing.
The dual problems of hard starting and lousy suspension seemed to make the 1999 Husqvarna TE 410 totally useless.
As much as I disliked the 1999 Husqvarna TE 410 suspension I did persist with trying to get it started for quite a few days. Taking the spark plug out and cleaning it with the propane torch did the trick every time. Just what gasoline was in the tank also seemed to make some difference, as some days I did actually get it to kick start a few times while out riding around. Then one day the gasoline was dramatically lower energy density garbage that would barely run. I tried it in a few other bikes, and there was increased popping out the exhaust under deceleration, a weak lean feeling at all throttle openings and all engine speeds, quit a bit of surging and an unusually low and unstable low idle. In the 1999 Husqvarna TE 410 it was the same, huge amounts of popping out the exhaust on deceleration and a total lack of top end power despite very crisp operation and some little bits of surging around 6,000RPM or so.
On previous days the 410 hadn't been popping out the exhaust at all on deceleration, but then with this very weak low energy density gasoline it was popping out the exhaust on deceleration huge amounts. More even than the other bikes. What was also different was that I got the 410 motor to kick start several times in a row. It wasn't kick starting every time. Sometimes I would kick it and nothing would happen. Other times I would kick it and it would start and run low and weak for a split second, then stall and it wouldn't fire again. Sometimes though it was just firing right up on the first or second kick and running. Certainly a lot more kick starting than I had seen from the 410 on previous days.
The next day the gasoline was back closer to normal, the power was still very weak but not quite as weak, the popping out the exhaust was nearly totally gone and the 410 wouldn't kick start at all.
When I turned the idle stop out to lower the low idle speed the engine began popping out the exhaust a small amount on deceleration, and I was also able to get it to kick start a few times. Most of the time it still wouldn't kick start, but I did get it to fire up sometimes on the first kick after shutting it off.
Next I tried going in on the idle mixture screw, and this got more popping out the exhaust and also more kick starting. I kept going in on the idle mixture screw and the idling and starting performance kept improving. The popping out the exhaust kept increasing also. The farther I went in on the idle mixture screw the farther I was able to go out on the idle stop, and the low idle got more and more stable. Eventually it was actually able to idle without stalling, and I was getting it to kick start a lot of the time. It was also popping out the exhaust huge amounts.
Then something entirely unexpected happened. I turned the idle mixture screw all the way in until it was bottomed out, and that was where the engine idled best and kick started most easily. Wow, I wouldn't have expected that. Normally turning the idle mixture screw in on the DellOrto carburetors causes the engine to stall even once fully warmed up. Very strange indeed.
With the idle mixture screw bottomed out at zero turns out the low idle was able to be turned down very low and the engine low idled smoothly and consistently without stumbling or stalling. With that idle stop setting where the engine low idled down very low and stable the 410 motor also was able to reliably kick start nearly every time. There were still a few times when I would give it a kick and it would fire low and weak, stall and then wouldn't restart until I rolled it. Most of the time though the 410 motor was just firing right up on the first or sometimes the second kick and running without difficulty.
It seemed that what was going on was that the cranking and low idling spark energy on the Ducati Energia ignition system is so low that the spark plug won't fire through a dense intake charge. Only with the idle stop turned out substantially and a very lean mixture does the Ducati Energia ignition system have enough spark to fire reliably.
The funny thing though was that the starting and low idling performance was considerably different than on the 1991 and 1992 Swedish SEM CDI ignition systems. On the SEM ignition systems changing the idle mixture didn't make much difference in starting. If the SEM ignition wouldn't fire, no amount of turning the idle stop or the idle mixture screw was ever able to get the engine to fire up. The engines were still sensitive to idle mixture on the weak spark SEM ignitions in that using the choke when hot would usually prevent starting, and turning the idle mixture screw out farther than the stock setting also caused noticeably harder starting.
On the Ducati Energia ignition system though the low idle mixture was seeming all critical. The engine just wouldn't start at all with anything other than an extremely lean idle mixture.
My guess is that the Swedish SEM ignitions were providing a long duration weak spark at the cranking and idling speeds, and the Ducati Energia ignition system is providing a short duration but slightly stronger spark. It seems to me that a very short duration spark could be unusually sensitive to mixture ratios. A longer duration spark provides more time to get a larger number of molecules of gasoline to break down and burn, where the short duration spark relies on perhaps only a few molecules of gasoline being directly ignited by the spark. In either case the problem is a weak spark, but there seems to be more than one kind of excessively weak spark.
There are more clues about what is going on with the Ducati Energia CDI ignition system also. I tried the other Ducati Energia CDI control module that came with my 1997 TE 610 motor, and that one wouldn't kick start either. It seemed exactly the same in terms of starting performance as the 1999 TE 410 Ducati Energia CDI control module. I did get it to kick start once with the 1997 module, but it wouldn't repeat that performance. On that day it would give some low little weak pops sometimes when trying to kick start it, but it essentially never actually started with the kick starter. It roll started very easily on either CDI module, and the idling performance seemed the same also. What was different though was that the 1997 TE 610 Ducati Energia CDI control module wasn't advancing as far, and the 410 motor wouldn't run at higher engine speeds with less spark advance on that weak gasoline. Down bellow about 3,000RPM both CDI modules seemed identical, the same hard starting and the same unstable low idle with stalling. That was before I turned the idle mixture screw down to the bottom.
It's not an isolated fault with one CDI control module. All the Ducati Energia CDI modules appear to have weak spark, at least all two of them that I have. All of two is considerably more compelling evidence than all of one, but it is still not exactly a large sample size.
The other big clue is that although the 1999 Husqvarna TE 410 usually seemed very close to kick starting, and would sometimes even give a few pops, dramatically reducing the spark plug gap didn't appear to help in the slightest. When I tried a 0.011" plug gap it was a different spark plug, and the 410 still wouldn't kick start. When I roll started the bike though it fired up just the same and ran very similarly with hardly any bobbling or anything strange like that. I could feel a slight difference, but it was very slight. I shut it off several times, and each time it wouldn't kick start but fired right up when roll started. Then when I put the 0.023" gapped plug back in it was again just the same. It wouldn't kick start, but I did get some little low and weak pops every once in a while.
Something that was seemingly very strange was that sometimes I was able to get the 410 motor with the Ducati Energia ignition system to kick start by cracking the throttle open substantially. A few times it wouldn't start with the throttle closed, but cracking the throttle open got it to fire up with the kick starter. That doesn't sound much like a weak spark problem. It does sort of fit with the idea that the Ducati Energia CDI ignition system is delivering an excessively short duration spark though. The short duration spark is powerful enough to bridge the stock gap even with the throttle cracked open, but it doesn't last for long enough to get many molecules of fuel to ignite.
Despite the hard starting and crappy suspension I did like the 1999 Husqvarna TE 410. It's essentially just like my other Husqvarnas, but it looks different and has a unique color scheme. The original gas tank was gray, but one of the petcock mounting screws was stripped out. I replaced it with a black 1992 tank that happened to have. Not off of my 1992 Husqvarna, but just a used one that I saw advertised for a rather reasonable $40 delivered. With the black tank and the somewhat ugly 1994 and later Husqvarna four stroke radiator shrouds removed the bike looks sharp. It is a black and yellow seat with very prominent yellow fenders, and interesting combination of colors. A yellow and black striped Italian bike. I call it my Vespa.
My 1999 Husqvarna TE 410 was in fairly good overall condition. It had been ridden quite a bit, but mostly it was rather nice. There were however some minor issues that had to be dealt with. I decided to take the rear wheel off to grease the rear caliper pins, something that has often gotten overlooked on older dirt bikes. Sure enough the grease was old and dried up. At least water had not gotten in yet, so it was all still in pretty good condition. Some water proof grease on the caliper pins every once in a while goes a long way to keeping them in good condition. High temperature grease is a good idea on the caliper pins, although I have had fairly good luck with just any old water proof grease also.
The cables were bone dry, so I lubed them up also. While I had the cables off I replaced the clutch perch and lever with one I had off of a 1991 Husqvarna two stroke. The stock clutch perch on the 1999 TE 410 was missing the pivot bolt and also all the hardware for the manual compression release. The 1991 Husqvarna two stroke perch is made by the same Italian "Domino" company, it just lacks the mount for the manual compression release and the two stroke lever is a bit lighter. The Renthal dual compound grips were in great condition, they just needed to be glued onto the bars and throttle tube. I also cut the Renthal bars down to my proffered slightly narrower width for trail riding. The Cycra hand guards on the bike are pretty nice, and they have a matching yellow and black color scheme on the stickers. The mounts are bare aluminum colored and the guards are white, but the stickers have that yellow and black coloring that goes nicely with the rest of the bike. Taking all the controls apart gave me the chance to get some anti-seize on the fasteners and set it all up to my liking. The clutch pull on the 1992 and later Husqvarna four stroke clutches is rather light, but a well lubed cable and just the right adjustment helps even more.
I had taken the clutch cover off to check on the oil reed valve while I had the top end off, so I also got a look at the clutch. Even though the 1999 TE 410 shows signs of quite a lot of use, the clutch basket was like new. Not the slightest bit of pitting. That's nice, I wonder how long it will stay that way. The clutch cover had been off many times before, and the old gasket was torn up something awful and leaking a bit. At least there was some gasket there to work with. Cleaning everything up and coating both sides with a light smearing of RTV silicone got it to go back together nicely without leaks. I often run no clutch cover gasket at all as they have been a bit hard to come by at certain times, and this does sort of work. If both sides are well cleaned and evenly coated with a light coat of RTV silicone they often don't leak. With no gasket though those long spaces between bolts do sometimes develop small seeping leaks, which can be annoying. A new gasket is a really good idea if it can easily be obtained.
Another thing that was wrong with my 1999 Husqvarna TE 410 was that several of the Zerk fittings were broken off. Not missing, but broken off. And the broken off stubs were rather stuck. I got the broken stub out of the main linkage out with an easy out, but the broken Zerk fitting on the pivot clamp bolt just wouldn't come out. I broke two easy outs trying to get it out, but no luck. When I broke the second easy out off the piece stuck fast in the broken off stub in the end of the bolt, and I couldn't get it out. The easy out was so hard that I couldn't drill it at all even with an expensive high alloy drill bit.
I did get the pivot clamp bolt repaired so that both Zerk fittings work, but it took some drastic measures. I burned a hole through the easy out and broken off stub of Zerk fitting using the cutting torch, and then I brazed a Zerk fitting into the gaping ugly hole. This worked, although future Zerk fitting replacement on this particular clamp bolt is going to be less than straight forward.
The linkage and swing arm pivot bearings were good, and they should stay good for quite some time getting regular greasings. Amazingly those bearings seem to never wear out when they get greased sometimes. It is only when the Zerk fittings fail or are severely neglected that the bearings dry out and fail. I have even run swing arm pivot bearings without seals on them, and they lasted seemingly forever if I gave a small squirt of grease at oil changes or after particularly wet rides. On my 1999 TE 410 the bearings are good and the seals all hold grease. Very nice.
Other minor problems centered around loose bolts. The sub frame mounting bolts were both loose, and most of the muffler mounting bolts also were somewhat loose. This gave the whole back of the bike a loose and wobbly feel and sound. Tightening all those bolts up got the exhaust system to stay in place without wiggling around.
The previous owner said the one thing he did to the bike was replace the falling apart air filter element with a new Uni brand element. What I saw of the element through the little holes in the air box looks good. When I poked a stick in to check the oil level on the element it seemed somewhat over oiled. There might be a bit more performance to be gained by squeezing the oil out properly, but then again it might not actually make any difference on the smaller displacement 410. I generally find that air filter maintenance is more critical on the 610 motors than on the 350, and the same is probably true of the 410. It is a pretty big element, even the 1992 and later element that is very slightly smaller than the 1991 element.
I've got the 1999 Husqvarna TE 410 all fixed up and ready to go now, but it's hard to start and I don't want to ride it anyway because the suspension is bad. At least it looks good.
One of the most interesting things that has come from my recent 1999 Husqvarna TE 410 project is that I have figured out what is different about the 1999 through 2004 Husqvarna four strokes. I had been noticing about a year that Pro-X now makes pistons for the Husqvarna 610 motors. The confusing thing was that the Pro-X Husqvarna 610 pistons were always listed as being for 1999 through 2004 Husqvarna 610 motors, and not for 1991 through 1998 Husqvarna 610 motors. I wasn't sure what was going on with that, but now I know that the difference is that the connecting rod was lengthened for the 1999 model year. I have even seen replacement 1999 through 2004 Husqvarna 610 connecting rods advertised for sale recently, and they are listed as having a 132mm center to center length and a 39mm big end bore on the same 30mm crankpin. That's 5mm longer center to center than the 127mm connecting rod that's in the 1990 through 1998 Husqvarna four stroke motors. Mystery solved. The Pro-X pistons only go in the 1999 through 2004 Husqvarna 610 motors because they have the shorter compression height for the longer 132mm connecting rod. That also explains where that strange 24.8mm compression height Vertex brand piston that was in my 1997 TE 610 motor came from. It was probably a Husqvarna 610 piston for a 1999 to 2004 Husqvarna 610 motor, and when it was installed in the 1998 motor on the stock 127mm connecting rod the compression ratio came out way down at a completely unrealistic 7:1. What intrigue!
With the same 30mm crankpin and the same bearing width the capacity of the rod bearing is about the same. The larger 4.5mm rollers and 39mm big end bore does slightly increase the load carrying capability of the bearing, but ultimately it is the 30mm crankpin diameter that is most significant. Bigger rollers might work a bit better under some circumstances, but they also make the big end of the rod larger. The difference between the 38mm big end bore of the 1990 through 1998 Husqvarna connecting rods and the 39mm big end bore of the 1999 through 2004 Husqvarnas is however insignificantly small. It's just one silly millimeter and the overall weight of the rod comes out very close to the same.
What does strike me as very significant though is the length of the 1999 through 2004 connecting rod. The longer rod means that stroking the motor out to 610cc is much more practical. If the Husqvarna 610 motor with the original 127mm connecting rod is stroked out to a 3.18" stroke length for a 610cc displacement on the 98mm bore then the rod to stroke ratio gets very short at 1.57:1. That's not unheard of short, but it is too short. The stock 3.01" stroke length 577cc Husqvarna 610 motor with the original 127mm rod already has a rather short 1.66:1 rod to stroke ratio, and going shorter than that does not seem like an upgrade. With the 1999 through 2004 Husqvarna 610 rod's longer 132mm center to center length though the 3.18" stroke length for a full 610cc displacement yields a much more appealing 1.63:1 rod to stroke ratio. The longer rod makes stroking the motor out that little bit farther look like a much more realistic project. And there is enough room in the cases to go out to 3.18 inches of stroke length even with the slightly larger big end shell diameter of the long rod. Since it is only the narrow center portion of the rod that is bigger the oil flow issues seem less critical. The stepped rod tends to push the oil out of the way more easily when the cases are a bit too close. It sure seems like someone at Husqvarna was thinking along these lines back in the late 1990's.
As far as the stock 98mm bore and 76.5mm stroke length 577cc 1999 through 2004 Husqvarna 610 motors go there are some interesting piston options available from Woessner. Husqvarna called the 98mm bore and 76.5mm stroke length kick start motor a "570" model from 2002 through 2004, but it is mostly identical to the 1999 through 2001 kick start Husqvarna 610 motors and the pistons and rods are fully interchangeable. The stock replacement 10:1 piston is the 5.4cc (0.330cu in) dished Woessner 8526DA which has an advertised weight of just 319g. Since the longer rod does tend to add a few grams the considerably lightened dished short compression height piston is a nice upgrade. There is also a high compression option available from Woessner for the 1999 through 2004 Husqvarna 610 motors. The Woessner 8568DA piston has an 8.2cc (0.500cu in) dome to bring the compression ratio up to a whopping 12.4:1. The 8568DA domed piston is considerably heavier at 340g, which indicates that some of the dome can simply be cut off if a slightly lower compression ratio is desired. That 8.2cc of aluminum weighs 26g, so cutting the entire dome off to make a flat top piston would result in a 315g weight. It might not be possible to do that, but it is likely that 1/3 of the dome volume could easily be removed to reduce the compression ratio down to a more moderate 11.8:1 that would tend to make tuning a bit easier when using very low pressure pump gas.
Why Woessner doesn't offer a flattop piston for the 1999 through 2004 Husqvarna 610 motors is a bit hard to figure out. There are however both stock type 10:1 and high compression 11:1 pistons available from Pro-X, so all the bases seem to be covered for these later 1999 to 2004 Husqvarna 610 motors. There was also the flattop 24.8mm compression height Vertex brand 98mm piston which might still be available.
The 8.2cc pop up dome Woessner 8568DA piston could be cut down somewhat more substantially if material was taken off of the entire top of the crown, but this is a much more substantial machining operation than simply lopping off a chunk of the top of the dome for a slight compression ratio reduction. With crown thicknesses of around a quarter inch flattop pistons and dished pistons can easily be milled down for some rather substantial compression ratio reductions. In the 98mm bore Husqvarna 610 motor taking 1/32" off of the top of the crown lowers the compression ratio by about one point. More when starting at 12:1 and less when starting at 10:1, obviously. Specifically it would be 12.0:1 down to 10.9:1 or 10.0:1 down to 9.2:1 if there weren't any valve reliefs. Since the higher compression ratio pistons have bigger valve reliefs it ends up being much closer to a one point compression ratio reduction for 1/32" off of the crown regardless of what the compression ratio started out at.
The thinner crown doesn't have as substantial heat dissipating capabilities, but the stated purpose of lowering the compression ratio is normally to obtain a milder state of tune that isn't going to put as much heat in the piston anyway. Flat top pistons are very easy to cut down by 1/64" or even 1/32" for a slight compression ratio reduction and even dished pistons are fairly easy to cut down using a four jaw chuck in a lathe, a turn table on a milling machine or a CNC milling machine. Cutting the entire top of the crown on a pop up dome piston on the other hand is a much more substantial custom machining operation. The easiest way to cut the dome on the Woessner 8569DA piston would be to chuck it in a lathe and follow the same profiles as the stock Woessner dome. The huge valve pockets then get smaller, so more than 1/32" of material has to be taken off to get that one point of compression ratio reduction. What it comes down to is that domed pistons are simply more difficult to dramatically modify than flat top or dished pistons, even if there is still plenty of "meat" on the crown for some significant compression ratio reductions. When it is only a small compression ratio change that is desired though just the dome itself can probably be expeditiously lopped off in one fell swoop. That's only going to be a half a point or so though. Not enough to bring the monstrous 12.4:1 Woessner 1999 through 2004 Husqvarna 610 compression ratio down to the 11:1 Woessner 1990 through 1998 Husqvarna 610 compression ratio.
If a more friendly 10.6:1 or 11.0:1 compression ratio on the 1999 through 2004 Husqvarna 610 motors is desired with stock or mostly stock Woessner pistons then base gasket changes would probably also be required. Then changing the base gasket thickness means that the cam timing will change slightly unless it is reset back to stock. Adjusting the cam timing on the centrifical de-compressor equipped camshafts isn't as easy as cam timing adjustments on the 1993 and earlier camshafts. The later type sprockets don't fit as securely on the camshaft which means that they rely more on the bolts for centering. Elongating the bolt holes on the hub centric 1991 cam sprockets is very easy and works well, but elongating the bolt holes on the 1994 and later cam sprockets is more problematic. It can be done, but it is trickier and more likely to cause a failure.
What this all adds up to is that there seems to be a hole in the Woessner piston offerings for the 1999 through 2004 Husqvarna 610 motors. It's really a very big jump up from a 10.0:1 compression ratio with the dished Woessner 8526DA piston to a 12.4:1 compression ratio with the Woessner 8568DA domed piston. In my experience that large of a compression ratio difference on the Husqvarna 610 motors is the difference between 31 degree BTDC spark timing and 24 degree BTDC spark timing on high pressure gasoline. On low pressure gasoline that much of a compression ratio difference is even more dramatic, and is the difference between 26 degree BTDC spark timing on the low compression ratio engine and about 16 degree BTDC spark timing on the high compression ratio engine.
When the compression ratio gets very high up above about 11:1 on low pressure gasoline just what spark timing value appears to work best gets somewhat unpredictable. A high compression ratio engine running low pressure gasoline may be sufficiently crisp with 12 or 14 degree BTDC spark timing, but it is pretty easy for that small of an amount of spark advance on a four inch bore engine to cause severe stumbling and cutting out problems if the tuning is not extremely precise. Even if a high compression ratio four inch bore engine is sufficiently crisp and will make power with 12 or 14 degree BTDC spark timing most tuners would end up picking more like 15 to 18 degree BTDC spark timing just to get the stumbling and cutting out to go away. The extra spark advance causes harsh operation and may even reduce torque production, but if the engine won't run at all then it isn't good for much. When running a fixed advance curve (no load dependant spark timing mechanism) spark timing later than about 15 degrees BTDC is pretty much out of the question. If the spark timing is ending up that late then the compression ratio probably needs to be lower. It certainly is possible to go all the way down to 10 degree BTDC fixed spark timing, but not desirable under most circumstances.
When a four inch bore 10:1 motor won't make torque from 3,500 to 5,000RPM with 24 to 28 degree BTDC spark timing because of excessively high pressure gasoline then substantially higher compression ratios will work on the same gasoline. All the way up to about 12:1 or even 13:1 with a fixed spark timing value somewhere around 20 to 23 degrees BTDC on the same four inch bore.
A competent load dependant spark advance mechanism (not vacuum advance) can work with high compression ratios even on somewhat less spectacularly high pressure gasoline, but it has to be setup for the gasoline that is actually used. Even in the face of highly variable gasoline supply or when an engine is used over a very wide range of altitudes without other adjustment, a load dependant spark advance mechanism can be useful, but it ends up being more like an operator controlled spark timing adjustment than true load dependant spark advance. The problem with the TPS as an operator controlled spark timing adjustment is that excessive harshness at small to medium throttle openings tends to be a pervasive characteristic of low altitudes and/or low pressure gasoline. Only when the load dependant spark timing mechanism is setup for the gasoline that is actually used is it able to function as a true load dependant spark advance mechanism.
If the 12.4:1 compression ratio of the Woessner 8568DA pop up dome 1999 through 2004 Husqvarna 610 piston seems too high for a fixed advance curve on pump gas then the question does come up: What is it good for? Race gas of course. Traditionally compression ratios of around 12:1 to 13:1 were considered ideal for the most powerful race gas normally available. But there is also another perspective on the Woessner 8568DA piston. What is the absolute easiest way to build a big displacement Husqvarna 610 stroker motor? Of course, just slap a Woessner 8568DA pop up dome 1999 through 2004 Husqvarna 610 piston on a 127mm 1990 through 1998 connecting rod and move the crank pin out to get the compression ratio high enough. If the stock 10:1 compression ratio is the goal then it's a lot of displacement indeed. The rod to stroke ratio gets way too short, the clearance between the rod and the cases gets very small and the rod bearing itself looks very small, but it might work anyway. The roller rod bearings can handle immense loads when they are well lubricated, so it might just be possible. The shortest rod to stroke ratio that was ever used on automotive engines was 1.53:1. That's way too short, but it has been proven to at least sort of work. If that 1.53:1 rod to stroke ratio was used with the 127mm connecting rod then the stroke length could be all the way out at a massive 83mm (3.27"). That seems like way too much for the Husqvarna cases, but it does just barely fit with just a tiny bit of clearance left over so that the rod doesn't physically smash into the divider at high engine speed.
The absolute maxi 83mm stroke length with the stock 98mm bore is a whopping 626cc of displacement. With the Woessner 8568DA pop up dome 1999 through 2004 Husqvarna 610 piston and the 127mm rod on that 83mm stroke length works out to a 10.8:1 compression ratio. If a stock compression height 1999 through 2004 Husqvarna 610 piston, such as the Woessner 8526DA, is used instead the compression ratio of the 626cc maxi stroker comes out way down at 9.1:1. Very low by modern motorcycle standards. When stroking an engine out on the same bore the compression ratio can stand to be a small bit lower. The larger displacement puts more heat into the cylinder head at the same engine speed, so it is easier to get late compression ignition with moderate amounts of spark advance on a slightly lower compression ratio. Still though, 9.1:1 on a water cooled 98mm bore engine is way down there.
That's the easiest way to build a big displacement 98mm bore Husqvarna 610 stroker motor, but not necessarily the best way. The easy build with the 127mm connecting rod means that the parts all go together bone stock without any modification other than moving the crank pin out. It also means that the rod to stroke ratio is horrendously short. Much better would be to build a 610cc stroker motor using the longer 132mm 1999 through 2004 connecting rod. The challenge with the long rod 610 stroker motor build is stuffing everything in. There tends to be substantial interference between the piston and the cylinder head, so some adjustments need to be made. The OEM 1999 through 2004 Husqvarna pistons are probably better for this purpose. They are heavier with thicker crowns than the Woessner pistons, so there is more material to work with. This long rod 610cc Husqvarna 610 stroker motor build requires that many small things be changed to get the required clearance and to get the compression ratio down low enough. If a massive 0.050" is taken off of the OEM 1999 through 2004 Husqvarna 610 piston and an additional 0.020" of base gasket thickness is added then the piston crown clears the cylinder head gasket sealing surface generously and the compression ratio comes out at 11.0:1 with no modification of the cylinder head.
If a lower compression ratio were desired, or if less material was to be taken off of the piston so that the Woessner 8626DA piston could be used then material could be taken out of the combustion chamber. The material that needs to be removed around the outsides of the valves to reduce shrouding is a miniscule 1cc which would reduce the compression ratio by less than a quarter point. There is however plenty of room to remove extra material from the combustion chamber. The problem with opening up the combustion chamber more is that it is a one way modification. It is easy to take material off of the combustion chamber, but it is much more difficult to put it back on if it is later decided that the compression ratio came out too low.
A reasonable strategy might be to open up around the valves very aggressively to provide the absolute best possible flow and then cut the piston down only as far as required to set the desired compression ratio. If later a higher compression ratio were required then it would just be the piston that would need to be replaced.
In any case building the 610cc 3.18" stroke length Husqvarna 610 stroker motor with the long 132mm 1999 through 2004 connecting rod is entirely possible; it just takes some custom engine building. The payoff compared to the 127mm rod easy-to-assemble stroker build is a much more reasonable 1.63:1 rod to stroke ratio, more clearance between the big end of the rod and the cases, a more realistically sized big end bearing and the compression ratio can be set anywhere from 10:1 to 13:1 as required for the gasoline being used.
The 3.18 inch and 3.27 inch stroke lengths are very long for practical gasoline engines, but on hot burning race gas there is still plenty of power production potential out to 8,000RPM and beyond. These stroker motors clearly are more powerful than the stock 3.01 inch stroke length 577cc Husqvarna 610 motor, but with the stroke length out that long the advantages of more displacement on the same bore diameter are dubious at best. There is little reason to think that the 3.27" stroke length short rod stroker motor would have any advantage in racing over the 3.18" stroke length long rod motor. It's only two and a half percent more displacement and the rod to stroke ratio is perilously short. The long rod motor would tend to be more reliable and would also tend to run better. The 3.18" stroke length 610cc Husqvarna 610 stroker motor using the longer 132mm 1999 through 2004 rod clearly is a better build strategy.
To use a somewhat more moderate stroke length with the short 127mm 1990 through 1998 connecting rod there is also the possibility of swapping timing sets. The 1994 through 2004 Husqvarna 610 motors use an 18/36 timing set with 5/16" chain, where the 1990 through 1993 Husqvarna 610 motors use a 3/8" timing set of the same total height. All of the 1991 through 2001 Husqvarna 350 and Husqvarna 410 motors usa a 5/16" timing chain with smaller 17/34 sprockets. What is significant about those smaller sprockets is that they could potentially be used to lower the cylinder head on a Husqvarna 610 motor by 0.078" (2mm). Switching from the 18/36 sprockets to the 17/34 sprockets actually acts to raise the cylinder head by 0.234" (6mm), but removing two links of the 5/16" timing chain lowers the cylinder head by 5/16" for a net reduction in clinder head height of 0.078" (2mm).
What this means is that a less extreme stroker motor could be built using the short 127mm 1990 through 1998 connecting rod. The 3.18" stroke length for the 610cc displacement with the 0.078" lower cylinder head position would yield a somewhat low 9.6:1 compression ratio with a stock 1999 through 2004 24.1mm compression height 98m piston. That 9.6:1 compression ratio would be with the cylinder cut down by 0.078" and on the stock 0.050" base gasket. If a bit more were cut off of the cylinder, or if a thinner base gasket were used, the compression ratio could be brought up somewhat, but the timing chain would tehn be hanging rather loose.
SInce the Husqvarna 610 timing chains usually hang rather loose with the stock base gasket it seems reasonable that the cylinder head might be able to be moved up by a small amount. The shorter 350/410 timing set might just barely fit on the stock length cylinder with no base gasket, which would lower the compression ratio even more. If the pop-up dome 98mm Woessner 8568DA 1999 through 2004 Husqvarna 610 piston was also used then the compression ratio could be increased substantially. The stock length cylinder on this 610cc stroker build would yield a 10.8:1 compression ratio with the pop-up dome Woessner 8568DA piston and no base gasket. And that would be a rather fixed compression ratio since the timing chain would be stretched out as far as it will go and there is no base gasket to remove.
The rod to stroke ratio is still an excruciatingly short 1.56:1 on this 610cc short rod engine, but it is certainly a milder and more practical stroker build than the monstrous 83mm stroke length 626cc maxi stroker. Th timing-set-swap stroker build remains simple in terms of using an off the shelf piston and an entirely stock Husqvarna 610 cylinder head. And with the pop-up dome Woessner 8568DA piston the stock length Husqvarna 610 cylinder can even be used.
The long rod 610cc 3.18" stroke length stroker build is still better because the rod to stroke ratio comes out more reasonable, but the short rod 610cc 3.18" stroker build is appealing because it does not require modification of top end parts. A project that just bolts together tends to be much more approachable.